The invention relates to a luminescent screen comprising a bearer provided with a luminescent material comprising a luminescent zeolite containing rare earth ions.
The invention also relates to luminescent zeolites for use in such a luminescent screen and a method for preparing such zeolites as well as to a low pressure mercury discharge lamp comprising such a luminescent screen.
A luminescent screen as described in the opening paragraph is known from international patent application WO 95/16759. In general a luminescent screen is used to convert excitation energy into radiation with a certain range of wavelengths. The excitation energy can for instance consist of an electron beam, X-rays or UV- radiation of relatively short wavelengths. Among many other applications luminescent screens find application in lamps, particularly in low pressure mercury discharge lamps or fluorescent lamps. The excitation energy then mainly comprises UV-radiation of approximately 254 nm generated by mercury present in the plasma of the fluorescent lamp. The composition of the luminescent screen is chosen in dependence on the desired spectral composition of the light radiated by the fluorescent lamp. In the known luminescent screen the rare earth ions contained in the zeolite are Ce.sup.3+ ions. When the known luminescent screen is excited by UV radiation with a wavelength of 254 nm it emits light having an emission maximum at a wavelength between 300 and 400 nm. Such a luminescent screen can for instance be used very effectively in low pressure mercury discharge lamps for use in photochemical processes, such as polymerization, lacquer hardening, drying, curing, medical irradiation purposes or suntanning. The quantum efficiency of the zeolite in the known luminescent screen is very high. Because the rare earth ions are enclosed by the zeolite structure the known luminescent screen is also very stable in different chemical environments and within a wide temperature range. Zeolites are also generally very cheap and environment-friendly. Most of the time, however, it will be desirable that the fluorescent lamp radiates light in the visible part of the spectrum. Such an emission in the visible part of the spectrum can be realized by the incorporation of other rare earth ions in the zeolite such as e.g. Tb.sup.3+ ions. When a luminescent screen provided with a luminescent material comprising a zeolite containing Th.sup.3+ ions is excited by means of UV-radiation with a wavelength of 254 nm, visible radiation with wavelengths within the range 450-650 nm is emitted by the luminescent screen. A problem, however, that is associated with many zeolites containing rare earth ions emitting visible light upon excitation with UV-radiation with a wavelength of 254 nm, is that the absorption of the exciting radiation is very low so that the amount of visible light that is generated out of the exciting radiation is relatively low.